ABSTRACT Despite decades of research into the immunopathology of heparin-induced thrombocytopenia (HIT), an unpredictable, life-threatening, immune-mediated adverse reaction to heparin treatment, a fundamental knowledge gap regarding the cause of HIT remains. The inability to predict HIT represents a considerable liability associated with heparin administration. Because the exact cellular and molecular mechanisms underlying HIT have yet to be identified, including the impetus for antibody production and critical immune cell roles, there is an essential need to apply alternative approaches to understand the biological basis for HIT and to identify clinically implementable biomarkers. The research objective of this proposal is to determine the role of genomic and transcriptomic variation in HIT pathogenesis. The PI's central hypothesis is that variation in genomic and transcriptomic pathways impacts HIT pathogenesis and can be used to identify clinically implementable HIT biomarkers. Based on strong preliminary data that constitute the first genome-wide association study (GWAS) and N-of-1 pathways studies for HIT, the working hypothesis is that the upregulation of T cell activation pathways results in PF4/heparin antibody production and that histo-blood group gene ABO O alleles subsequently result in HIT through platelet activation. We will pursue two Specific Aims (SAs) to test the central hypothesis: (1) determine the influence of genomic variation on HIT and (2) identify transcriptomic pathways involved in HIT pathogenesis using an N-of-1 strategy. In SA #1, a GWAS and gene resequencing approach will be utilized to identify genetic associations with HIT and platelet factor 4 (PF4)/heparin antibody production. In SA #2, peripheral blood mononuclear cells (PBMCs) will be collected and RNA-Seq will be performed. The N- of-1 pathways approach will be utilized to determine up- and down-regulated transcriptomic pathways involved in HIT and in the development of PF4/heparin antibodies. The proposed research has the potential to garner significant insights into HIT pathogenesis and to provide a framework for the clinical translation of HIT biomarkers into early diagnostic and preventive strategies. The proposed studies are technically innovative as they leverage novel strategies for large-scale biological data, including the N-of-1 pathways approach, and because establishing tools to distinguish patients that are pre-disposed to HIT could potentially shift current clinical practice paradigms. The PI's career development objective is to acquire expertise in bioinformatics and the research tools necessary to address his long-term research goal, including experience in big data, genomic and transcriptomic pathways, and innovative clinical study approaches. The knowledge and skills garnered during this award will provide foundational training and resources for future work in pharmacogenomics, facilitate eventual submission of an R01 proposal, and foster the transition to an independent researcher investigating adverse reactions to cardiovascular drugs.